The prime minister said the intelligence around the situation was “gloomy” but war was not inevitable – urging Russia to pull back from a “disastrous step”.
Mr Johnson said Ukraine would “fight” if invaded, comparing the potential combat to the decades-long Chechen–Russian conflict. “It’s very important that people in Russia understand that this could be a new Chechnya.”
Speaking to reporters as the Foreign Office pulled some British embassy staff out of Ukraine capital Kyiv, he said: “We do think it prudent to make some changes now.”
The prime minister added: “The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see.”
Mr Johnson added: “We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step.”
Some British embassy staff and their dependants are being pulled out of Kyiv in response to the mounting Russian threat to Ukraine.
The Foreign Office confirmed the move after the US ordered the families of all American personnel at the US Embassy to leave the country in response the the risk of an invasion.
Russian forces have massed at the border with Ukraine and intense diplomatic activity has failed to ease tensions.
The prime minister said he would speak to fellow western leaders later on Monday, and claimed the UK was “leading” on the west’s package of economic sanctions against Russia should they invade.
“We need to get over the message that invading Ukraine, from a Russian perspective, is going to painful, violent and bloody business,” Mr Johnson warned.
He said: “I’ve been the Ukraine several times, I know the people of that country a bit. My judgement is that they will fight. And really, that is not the way the world should be going. I hope they understand that in the Kremlin.”
Mr Johnson added: “But it’s the job of the UK to make sure our friends and partners around the world, particularly in Europe, also understand that, and we get ready a tough enough package of sanctions.”
It follows foreign secretary Liz Truss’s accusation that Mr Putin of plotting to install a pro-Moscow leader as head of the Ukrainian government.
The Foreign Office took the unusual step of naming former Ukrainian MP Yevhen Murayev as a potential Kremlin candidate to take over in Kyiv – a claim dismissed as provocative “nonsense” by Moscow.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies